a couple of questions...

I have four plants on day 12 or 13 in flowering, what point in life does root development stop, or does it? At what time in its life does it stop growing(heigth) and concentrate on flowering?

not sure on root development but dont think it ever stops developing roots.
most plants 3-4 weeks into flowering will stop growing and concentrate on bud development but some long flowering strains may continue to grow for longer.

Root development will keep on as long as the plant is alive and has space to grow its roots.
Your plant should become rootbound just about the same time you are ready to harvest it.
A plant that IS rootbound becomes unhappy real quick and will tell you so... nutrient problems will become more pronounced, and you will see the leaves sort of drooping along their length, like umbrella shaped sort of.
The plant always has to make new roots to replace old tired or dead roots.
Don't worry though because the plant is still concentrating on bud development anyway as long as it isn't trying to grow new LEAVES.

Thanks for your help.

Anytime.